As is well known, world energy consumption is increasing at an alarming rate while the supply side of the fossil fuel has limited reserves. In addition to fast depletion of the projected fossil fuel reserves (petroleum, gas and coal), their irreversible damage to the environment and atmosphere (greenhouse gas) prompted exploration the environmentally friendly, alternative energy sources. Since mid the 1970, many studies were undertaken by academia, the private sector and governments around the world looking for environmentally friendly alternative energy sources. From these studies, natural resources like the wind, solar, ocean waves, currents and biomass energy-generating technologies emerged. Especially, wind power turbines became the most accepted energy source and is growing at a rate of 20% for year over world-wide. Even with the large advancement in these technologies, the total electric generation from these sources, excluding hydroelectricity, is still less than 3% of the world total energy consumption today.
While the potential from natural resources including solar and wind are much higher than the world power consumption, harvesting the useful energy from these sources in a steady way is very challenging. However, required technologies, at least for the wind and the solar farms, are well developed, their prices are coming down, and utility scale power plants are in operation. With all of these improvements, the biggest problem is that these sources are not predictable. For example, in general, the power generation from a wind farm dies off during the daytime, especially during the afternoon hours when electricity is needed the most. However, it picks up or reaches its maximum during night hours when electricity is needed the least. Similarly, solar or ocean wave based energy generation process is not predictable either. Battery based storage technologies are still in development stage and they are too expensive in addition to their short power discharge life. Underground compressed air energy storage systems (CAES, only two systems in operation) are reliable but they are also very costly to construct. As seen from this discussion, new, innovative, safe, efficient and inexpensive energy storage facilities for gas and liquids are urgently needed. This is necessary for not only for the survivability of renewable energy sources but also for them to be considered as serious alternatives to fossil fuels. Considering these problems, the deep water storage systems apparatus, may help to solve these challenging issues of renewable energy of sources, in addition to providing the environmental and human safety benefits.
The existing land based energy storage tanks used for storing compressed energy gas or liquids cannot be a comprehensive and long term solution for the energy storage problems, due to high risks to the surroundings (live-stock, hazard materials, fire). They also have technical limitations regarding the size of the tanks. These become very critical issues, especially when a power generation system requires large storage tanks for an optimum output from the generation plant. As is well known, internally pressurized larger tanks require high strength construction materials and larger wall thicknesses. Therefore, for safety reasons, and including many other factors, there is a limit to how big a tank can be constructed for given tank material(s) and internal pressure. These and many other problems associated with energy fluids storage are eliminated and/or at least minimized by deep water storage systems discussed in this invention.
Before going into the details of the present invention, it would be helpful to discuss some of the previous works and some parameters of a deep ocean that can be utilized for designing a novel sub-sea energy storage system. In the past, many theoretical studies were proposed to generate energy within sub-sea based systems. At first, these prior energy generation systems look conceptually very attractive. However, when you analyze them in detail, these prior power generation systems have many shortfalls. The most common problems are violation of the law of physics. Also, they may be impractical or require large capital investments and/or are based on unrealistic conditions. The required conditions cannot be easily met to build or to operate these sub-sea storage power generation systems. Some of them carry very fresh ideas while the overall system performance is either so poor or falls into the perpetual motion machine category (i.e., violates the first law of thermodynamics). For example, in the previous art (U.S. Pat. No. 7,911,073 B2), deep ocean water runs through water turbines located at the entrance of an empty water collecting tank secured at the sea floor while generating useful electric energy. This is fine, but to put the entire system in a steady state operation, those water collecting tanks have to be discharged into the ocean against the same static pressure that run the turbines in the first place. Discharging those tanks will not require less pumping energy than the turbines output. Similarly, in the previous art (U.S. Patent Publication No. 2000/0159892 A1), thermal energy is generated by allowing the deep ocean water to compress a working gas contained in a cylinder similar to a Pascal hydraulic system. Again, the amount of thermal energy generated cannot be greater than the energy needed to pump the water out or to empty the tank. In these examples and others, too often the inventors are not clear about running their systems in a steady state mode while still producing positive useful energy.
In order to give a better understanding of our approach to the large volume underwater energy fluids storage system, it is necessary to discuss the basic governing laws and main factors associated with deep ocean or lake waters. First of all, in a large body of water, there is a need for a potential difference between two points in order to generate energy or to compress a fluid(s). This potential could be differences in temperature (between two points), density, or pressure (weight per unit area) The higher the potential differences, the greater the chance of producing energy or some useful work. Regardless of the weather conditions and the location, the largest potential difference in deep water is the static pressure. It is a linear function of the depth. Having large potential differences in the body of water is the first necessary requirement. However, it is by itself not sufficient to generate energy. In order to produce a useful result (i.e., produce energy or do work), there is a need to put the fluid in motion between two points having difference in potentials. How to utilize this given potential difference in flowing water is dependent upon the method and the devices used in the system. If the system is steady flow or cyclical, useful energy in the form of electricity, thermal or mechanical works type energy per unit time (i.e., power) is produced. If the system is not cyclical, it is possible to produce energy one time only. In a continuous cyclical process, the net useful energy is determined by subtracting the input energy from the total output over a completed cycle of the system.
Large volume energy storage is the heart of the present invention. It is basically a novel, deep-water, stationary energy storage tank, which can also be utilized for high pressure energy generation such as H2 and O2 gases. This invention may use steady flow equipment such as pumps, compressors, heat exchangers and gas or water turbines at various stages of the cycle during energy storage. Hence, the basic equations of these devices are now discussed in order to provide a broader understanding of this invention.
When we apply the conservation of energy principle, or the first law of thermodynamics, over these steady-flow devices between inlet and exit (two port system, pump, compressor, turbines, etc.), the simplified version of the energy equation becomes as Q′−′=M′(h2−h1). This equation includes heat rate input (Q′) power output (W′), mass flow rate (M′) and the enthalpies (h2, h1) of the working substance at exit and inlet states. This is a simple steady flow energy equation and it is sufficient to analyze the energy balance of various devices used in the present invention. For the most part, during a compression or expansion process across pumps, compressors or turbines, the heat loss or gain by the substance can be neglected (except for the heat exchangers). What is left is the power output in terms of enthalpy changes. In this equation, if h2 is greater than h1, then the device is either a pump or a compressor; otherwise, it is a turbine (gas or liquid). As seen from this equation, it is possible to generate electric energy by allowing deep ocean water through water turbines at the entrance of an empty (initially) sea floor-based storage or receiving tank(s). Again, unlike a hydroelectric dam in which the water is running down after the turbines due to the lower elevation (gravity), here at the bottom of the ocean, once the tanks are filled, they need to be emptied against the same potential which caused the electric generation at the first place. The energy required to empty the tanks by pumps will not be less than the turbine outputs, although the power can be different due to the time frame. This issue, too, is often either skipped or not clearly discussed in many of the previous arts. This type of energy generating system must satisfy the first law of thermodynamics. Otherwise it falls into a perpetual machine category.
In the present invention, the sub-sea storage tank system is filled with energy containing fluids from a location at or near the surface of the large body of water. Using pumps, compressors, multi-phase pumps or hydraulic based compression devices, the fluids in the form of liquids or gases, including air, from a floating platform, a tanker ship or an offshore facility are transferred into the sub-sea storage tanks. As energy fluid occupies the upper region of the tank, it also replaces (pushes out) the sub-sea high pressure water which is already inside the tanks. The tank bottoms are either completely open or partially open, or they have large ports to allow the surrounding water to flow in and out of the tanks during the discharging and charging processes respectively. During the charging process, the compressors or pumps will require large power inputs regardless of the sources of the power whether a free or inexpensive energy from renewables during off peak hours or a direct pull from the utility grids. Therefore, a basic comparison analysis of the power consumption of these pumping devices will help to distinguish this invention while providing more clarity than the previous art.
First, considering the steady flow energy equation discussed above, the enthalpy (i.e., sum of internal energy and the flow work) change can be expressed in terms of other properties of a substance. For example, as is well known, for an incompressible substance, i.e., water, the enthalpy change is equal to the pressure change divided by the density. For an ideal gas, the enthalpy change is a function of temperature. As is well known, the classic ideal gas laws such as Boyle's, Charles and the Equation of the State can be utilized to obtain various relationships among the gas properties. These relationships are used in the conservation of energy equations for obtaining more practical power input or output expressions of these steady flow pumping devices. For example, for power calculations, the most commonly used relationships and/or assumptions are the equation of state and PV*n=C, in which if n=1 then T=C, if n=k=Cp/Cv then adiabatic and if n=n then polytrophic process. Special cases such as isentropic (frictionless adiabatic, an ideal process) and the isobaric (P=C) are also used, dependent on the process across the machine. In the present invention, the fluid handling equipment such as compressors, pumps and/or hydraulic systems (gas compression hydraulically instead of using a compressor) are used during the sub-sea energy storage operation. Therefore, a short mathematical discussion of the basic energy consumption of these devices including pump and compressor power inputs and/or a comparison analysis will further, assist in understanding the invention.
Assuming an adiabatic process, the ratio of the power consumptions between a compressor and the pump having the same mass flow rates and efficiencies (they are usually very close, 75%-85%) can be expressed: W′ comp./W′ pump=[Cp×T1×(Pr*(k−1)/k−1)]/[(g×H)/1000]. In this equation, Cp is the specific heat at constant pressure (KJ/kg-K), T1 is the initial temperature (° K), Pr is the pressure ratio (P2/P1) of the compressor, g is the gravitational constant (9.81-m/s*2) and H is the total head (m) of the pump and k is the adiabatic gas constant−the ratio of specific heats (for air 1.4).
As one can see from this equation, a compressor will consume more power than a water pump under the same pressure heads. For example, consider a generic air compressor and a water pump (i.e., single, double or multi stages, reciprocating/piston or centrifugal etc.). Air at atmospheric conditions (i.e., 1 bar and 27 C) enters into a compressor and it is compressed adiabatically to 10 bars or 100 bars. For the pump, water enters at atmospheric conditions and is pumped to 100 m height or 1000 m height (i.e., pump exit pressure is 10 bar or 100 bar). Further, assume that both devices have the same mass flow rates. Using the above power ratio equation, it can be seen that the compressor power will be 80 to 278 times higher than a pumping power for 10 and 100 bar exit pressures (compressor power relation is non-linear with respect to pressure ratio). Again, due to compressibility (a gas vs. a liquid), the compressor will use much more power input than the pump under the same mass flow rate and the same pressure head conditions. On the other hand, as an example, if we use a pump to accomplish a gas compression (i.e., a hydraulic system) instead of a compressor at the same volumetric flow rate of the compressor, the pump power consumption is higher than the compressor power. In the above example (air), the pumping power needed is approximately 3.5 to 12 times higher than the compressor power for 10 and 100 bar final pressures respectively. This is true due to the difference in densities between air and water, while keeping the pump swept volume per unit time the same as the compressor.
As mentioned previously, these qualitative comparisons of pump and compressor power consumptions are significant for the present invention, which may utilize both some or all of these devices. This is an important improvement over previous art which are too often either too weak or unclear about the net energy balance to operate their storage systems in a cyclic mode rather than a one time operation.
Also, existing land based energy fluid storage systems have many problems. They are not safe in case of any accident such as fire or leaks. They can cause huge damage to the environment, life and the surrounding property. Furthermore, they are costly to build and have technical limitations regarding how big the pressurized storage tanks can be built and still operated safely. As is well known, the higher the internal pressure, the higher the stresses on the tank construction material. Therefore, high pressure tanks require high strength construction material. The bigger the tank, the larger the wall thickness may be required. Therefore the land based large storage tanks are not low-cost solutions to energy fluid storage. As an alternative to conventional above ground storage tanks, especially for large volume storage needs, the underground storage facilities, especially using salt caverns, were explored for storing fossil fuels including the compressed gas. Some underground storage reservoirs along the main gas pipe lines are used temporarily. Despite the large storage capacities, whether underground or above ground, they are very expensive to build in addition to other strategic problems. Currently there are only two compressed air storage facilities (CAES) in operation around the world, both of which are land-based. One of them is in Alabama, USA, and the second, and oldest one, is in Germany. Another technical (operational) problem with (whether underground or above ground) is the pressure variation during the withdrawal of the gas for distribution or to run a power plant (i.e., for example a gas turbine). These pressure fluctuations, despite the use of smoothing devices such as diaphragm, regulator or a buster device, will have negative effects on the overall efficiency of the entire operation. As seen from these, a need for new, innovative storage facilities especially for storing natural gas, exists. The storage technology offered in the present invention provides substantial improvements over the existing energy storage may systems, apparatus, and methods, as one or more of the problems discussed above are either eliminated or substantially mitigated. The advantages include (but are not limited to) economy, reliability, safety, low cost construction and operation. Moreover, storage tanks configured and sited in accordance with the present invention may be capable of storing large volume of compressed energy fluids while providing significantly more security and safety than conventional land-based storage tanks.
One feature of the of invention disclosed herein is the use of new and novel methods and apparatus for storing very large volumes of energy fluids including compressed air, natural gases, and liquefied gases (LPG, CNG, LNG and others) within deep water, floor-mounted sub-sea storage facilities.
A further feature of this invention is a sub-sea storage system that can be extremely safe, out of sight of land, low cost to build and operate, and operate very efficiently, while capable of storing significantly large volume of compressed gases (especially for storing and transporting LNG in an extremely cost effective way) without building costly, land-based, insulated storage tanks and re-gasification apparatus.
Another feature of the disclosed invention is the compatibility with existing renewable energy generating systems including wind, solar and ocean current-wave based energy generating sources for storing energy from these sources.
Still another feature of the disclosed deep water energy fluid and system can be a low cost, safe and efficient loading and unloading and distribution terminal for compressed energy fluids instead of the existing land based or near-shore terminals/which are unsafe and environmentally dangerous in the case of an accident.
Since LNG has less environmental impact among fossil fuels, it is another objective of this invention is to provide a practical, safe and low cost and efficient alternative solution to the existing LNG land based loading/unloading terminals and storage systems.
Still another objective is to show that fresh water can be produced during the LNG storage process.
These objectives of this invention are accomplished in the following manner as explained briefly here in after.